Yesterday everything worked fine, I installed the updated plugin with imporved FSC and it worked great.

Today, not so much ;). When I try to run anything with or without FSC, I get: scala.tools.nsc.MissingRequirementError: object scala not found. Moreover, some random types are not "seen" by the editor, e.g. Option and Seq (they are highighted as red and auto-import suggest some Option from java). However, I can open Option by using "go to class". I tried recreating the project (using SBT), removing all the caches, removing the plugin and installing it again, but nothing helps.

The same problem happened to me. Adam, thanks for pointing to the source of the problem and the solution. In the dependencies tab for my module, the scala library was listed but the font was red and the icon was a globe instead of a bunch of books. I deleted that entry and added it again, after which it started working again.